The present invention relates to liquid level measuring methods and apparatus and particularly to methods and apparatus employing the use of transmitted and reflected microwave signals.
Prior to the present invention, one method of determining the level of a liquid in a tank using microwave signals was by measuring the time delay of a reflected microwave signal from the liquid surface in an coaxial delay line immersed in the liquid. The U.S. Pat. No. 3,853,005 to Robert E. Shendel, issued Dec. 10, 1974, and the references cited therein utilize this method.
In another known microwave liquid level measurement method, a multi-frequency continuous wave microwave signal is transmitted to a wave guide partially immersed in the liquid and the phase differences between the outgoing and reflected waves measured and used to determine the liquid level. However, in this and similar known methods, very accurate measurements and stabilization of the microwave frequencies are required.